A new kind of twin has been discovered, neither strictly identical nor fraternal. Now toddlers, the babies look identical but one is anatomically male while the other has ambiguous genitalia. Genetic tests show the children are identical on their mother's side but share only half their father's DNA.
Scientists theorize the "semi-identical" twins were created when two sperm somehow fertilized one egg, although that normally causes the cell to die. The sexual differences are a result of the odd number of chromosomes in the egg. (Read more gender stories.)